1. Field of the Invention
The invention relates to a solid state scanning device for automatic focusing and has particular utility in camera lens focusing. The invention is likewise closely related to my U.S. Pat. No. 3,918,071 entitled AUTOMATIC LENS FOCUSING METHOD AND APPARATUS, dated Nov. 4, 1975.
2. Description of the Prior Art
It is well known that images may be scanned by combinations of mechanical and optical apparatus of great complexity. When added to a 35 mm camera, for example, the bulk of a mechanical scanner and the required power supply represent a substantial part of the total. The investment in automatic focusing may well exceed that of the camera.
The moving parts of the scanner introduce vibrations, because a scanner rotates or pendulates at high speeds. These are principally a fundamental of high frequency and, experience teaches, many higher harmonics. Such vibrations will be translated into electrical noise equivalents by the scanner and will appear in the scanner output admixed to the image-derived frequencies. Thus, the output of the scanner becomes the sum of the image-derived frequencies, plus the vibration-induced frequencies. This composite is the input to the electronics which, by design, are particularly responsive to high frequencies. Thus the electronics will erroneously respond to the true input.
A further disadvantage of mechanical scanning devices derives from their scanning in circular patterns as shown in U.S. Pat. No. 3,776,639 of Stauffer. In most cases a rectangular format is changed. In the case of a 35 mm film frame, actually 24.5 .times. 36.3 mm., the circular scan may be made to inscribe itself in the rectangle so as to be tangent to two or three frame borders. Alternately it may be made to scan so that the scan circle circumscribes the format, with the corners of the frame located on the scan circle. In the first case only 53% of the image is scanned. In the alternate case, 100% of the image area plus 16% of an area external to the image are scanned. Assuming that during scanning of the format itself the unavoidable noise level results in a 100:1 signal to noise ratio, then the unwanted noise from the overscan will reduce the signal/noise ratio to 86:1 when integrated over both the image scan and overscan for a period of time. This represents a serious deterioration of the information.
One further deterioration of the signal will occur when abrupt changes arise as the scan becomes tangent to or traverses the image frame. This abrupt edge will induce high frequency transients of high value. Diffusion of the edges, reduction in scan diameter or other methods will reduce the scan area and thus decrease the information gained. Scanning into the frames will produce spurious high frequency response with the deleterious effects noted previously. As there is no present remedy, one of these conditions, as a minimum, must be tolerated in mechanical scanning.
Where scanning through a mask is employed, as in U.S. Pat. No. 3,776,639, about 50% of the information is lost as the scan sweeps across the opaque portions of the mask. U.S. Pat. No. 3,691,922 utilizes only a small amount of the energy from the image as does U.S. Pat. No. 3,532,045. The relatively low efficiency of partial scanning of the image will lead to poor performance in low light conditions specially.